TypeScript + Reducers

reducers.ts

type ThemeColor = string;
type Theme = { id: number, color: ThemeColor }
type State = { themes?: [Theme] }

type ExtractFirstArgument<T> =
    T extends (T1: infer T1) => infer R
    ? (T extends (t1: infer T1, t2: infer T2, t3: infer T3) => infer R
        ? Extract<T2, undefined> extends never
            ? () => R
            : (Extract<T3, undefined> extends never 
                ? (t2?: T2) => R
                : (t2?: T2, t3?: T3) => R
            )
        : any
    )
    : T extends (T1: infer T1, t2: infer T2) => infer R
        ? (T extends (t1: infer T1, t2: infer T2, t3: infer T3) => infer R
            ? Extract<T3, undefined> extends never 
                ? (t2: T2) => R
                : (t2: T2, t3?: T3) => R
            : any
        )
    : T extends (T1: infer T1, t2: infer T2, t3: infer T3) => infer R
        ? (t2: T2, t3: T3) => R : any;

// Se preferir deixar mais semântico a leitura
// visto que isso somente vai ser usado para reducers
type ExtractFirstArgumentForReducers<T> =
    T extends (state: infer T1) => infer R ? () => R :
    T extends (state: infer T1, payload: infer T2) => infer R ? (payload: T2) => R :
    T extends (state: infer T1, payload: infer T2, metadata: infer T3) => infer R ? (payload: T2, metadata: T3) => R : any;

const reducers = {
    setTheme(state: State, payload: ThemeColor, metadata: number): Theme {
        return {} as Theme;
    }
}

type Reducers = {
    [P in keyof typeof reducers]: ExtractFirstArgumentForReducers<typeof reducers[P]>;
}

const example = (reducers: Reducers) => {
    
}